What should I fill my raised bed garden with

Massgarden876April 1, 2014

So I am building my first raised bed vegetable garden (and also first garden ever). No experience, just read a couple books on sq. foot gardening. I think I have a pretty good idea of what I'm up against and I have a good feeling I will get a decent harvest.

The garden bed dimensions are 4'x12'x18", with one 4x4 section stepped up to 27" (for squash, melons, and pumpkins since they are deep rooted veggies). I have gone deep because the native soil is not ideal for vegetable growing.

Now... the one concern I have is getting the soil right. One book says (x) another says (y), one forum says (x) another says (y). You get the idea, there's a lot of mixed info out there. I know that "mels mix" is probably ideal but 3 cubic yards of mels mix is just not in my budget. I have access to a compost facility and I was planning on calling to get a delivery of 75:25 compost:loam, then adding some perlite and peat moss to the top 8-10 inches. Is this a good soil for this type of raised bed?

I would not add perlite into a garden bed though it does well in pots. The garden bed soil, since it's not compacted, should be light enough that water is going to drain well and allow roots to grow freely.

Peat moss absorbs a lot of water but it's difficult to get it wet again once it dries up outside a bucket, so it's not ideal near the top. If you add too much peat near the top, it might still be dry underneath though it looks moist at the top. It's it's more important to have the water be absorbed below and be dry near the top. I would not add more than 20% peat moss, and I would stir it throughout, not just for the top of the soil.

You might want to mulch the top with something to keep the soil from evaporating and keeping the surface dry at the same time. This will prevent disease. I use straw and this year I'm going to try bark nuggets.

The link below is to the same question just a bit further down the page here. There are many other similar discussions the search will pull up and there are even FAQs here about it as it is a very common question.

then adding some perlite and peat moss to the top 8-10 inches.

But as I mentioned in the thread linked below, do not layer your materials as that causes drainage problems. Mix all well together.

So then what about shorter raised beds? Say a 10" raised bed would say, be filled with a mels mix, then underneath it would be native soil. Would that not be layered? Or is it just the size that makes drainage an issue??

The drainage issue is simply when one layer doesn't allow the water to spread to the second layer evenly, etc. However, if you have the same type of material throughout the bed vertically, water would spread down evenly. So if you want to add perlite and peat moss, mix it throughout and not just at the top layer. There is no need for layers, or even much thinking as long as there is rich composted organic material in the soil (or whatever you are into), moisture retention, and good drainage.

Why is that ? Almost any native soil can be amendded to be and ideal soil. Adding compost, organic matter, manure and condition any soil from sandy to clay and anything in between. Soil is just a medium. You amend it to have a desireable drainage and moisture retention and then add nutrients that plants nedd.

But then there are cases that you do not have extra native soil to fill raised beds then you'll need to purchase the materials. That was my case last year to fill about 150 sq-ft raised bed of 11" deep. I purchase several cubic yard of soil sold a "garden soil". It was not really what I wanted, then I added more compost and manure. Last fall I added some more home made compost. NOW i am satisfied with it.

Making a good garden soil can take a couple of season.

Another point: Once your beds are filled, get a soil test to determine its pH. Right pH is viatally important for plants growth. Try to get a pH within 6.3 to 6.8. The last number being almost ideal.

Sounds pretty deep to me, but if you like the looks, go for it. When you're shoveling all that, you may regret it. Anyhoo, that deep one you might try filling with old logs to take up space. It's called hovelkulture or something like that. The logs hold moisture and then as they decompose will provide nutrients to your soil. I LOVE what Mel has done for gardening, but I think the Mel's mix is over the top. I haven't looked in his new book, but the one I got 20 plus years ago had about twenty ingredients, many of which someone in a podunk town like me would never find. Not to mention expensive, and would take you forever to recoup. For a lot of people gardening is like hunting, recreational. For me, it's fun, but I'm very much doing it to save on the ol' grocery bill. I'd say get as much compost as you can as cheap as you can, and use a nice 10 10 10 fertilizer to start with and slowly work your way toward organic. Maybe get you a worm bed started. I think initial success revolves more around staying reasonable on size rather than a perfect soil mix. The old saying is "don't plant a garden any bigger than your wife can handle."

Well I know my native soil isn't that great just by looking at it. I just moved in last year and have already revitalized my lawn with organic fertilizer and lime. From when I moved in, in the spring to the end of summer, the grass looked 10x better. I also used a brand new sharp mower and mowed high which made a huge difference).

Another thing is I realized when the snow melted recently, is that I have moles and voles. The last thing I need is to spend all this time and energy on this only to have it ruined by burrowers. I have put down chicken wire and permeable weed block at the bottom of the bed to compensate.

That sounds like a pretty interesting idea with the old logs. I have some seasoned fire wood. Is that acceptable?

Permeable means it s ability to allow liquids to pass through. Chicken wire is fine if you have moles/voles but landscape fabric is only mildly permeable. Testing consistently shows it slows drainage substantially. It is not ever recommended for the bottom of a bed by experienced gardeners and it would have no effect on the moles and voles anyway.

75:25 compost:loam is high on the compost side and the quality depends on your definition of "loam". Means many different things. But as long as it isn't so-called "top soil" or sand a 50/50 mixture of it would probably work. More commonly you see the recommendation for 40:60.

But you will have to add more compost at least once a year and better 2x a year as it continues to shrink as it decomposes.

I get the impression you aren't interested in first trying to improve your native soil and incorporate it into the bed, correct? That is the best route to go. But if not then you need to understand that overtime it can affect whatever you fill your bed with unless you made some sort of solid bottom box which is not at all recommended.

But you can still make it work with a framed raised bed on top of it with chicken wire or hardware cloth on the bottom IF you keep the bed well and frequently amended with quality compost. Over time the leachate and soil bacteria will improve the soil below the bed as well.

There is some good info on this thread...however, I must mildly disagree with above notions that hint all poor soils can become great soils with amendments. By it's nature....clay soil is composed of very fine/small particles which impede drainage. By adding larger particles ( for instance bark, wood, perlite, sand, etc ) the fine clay particles are not changed in any way. They remain. These finer clay particles still "clog" up the soil...filling in voids between the larger particles which were added as amendments. I do believe there is some value...but with time the "improved" soil will relax/compact and drain poorly once again. It is a battle. As was stated above..."stratifying" your garden with different layers is not ideal. In my neck of the woods, avocado growers add gypsum to clay soils to "break them up". The jury is still out on it's effectiveness. In short, I would much rather work in sandy soil than clay. I am not that lucky. There are many good articles/threads in "container" growing websites. Much of the information and principles can be transferred to growing in soil. Goodluck to all, and thanks for all the shared ideas.

I put the weed barrier down because there are a considerable amount of weeds on my lawn. I dig up te first couple inches of soil to level off the and recess the bed a bit. Is the weed barrier useless as far as blocking weeds? Will weeds even make it into my bed since I dig up the first few inches. I will simply take it out if it will have no effect on weeds and may improve drainage.

So I can get the delivery for $225 (anything under 8 yards). This facility is local and they claim to have the highest quality loam and compost in the area. Is this a good deal?

In one book I read it stated that 60:40 compost:loam was a good starting point if "mels mix" (of 1/3 vermiculite 1/3 peat moss 1/3 compost) was out of reach (which it is). Then I read on a forum, a fellow who used 100% compost and yielded his best results ever by far. So I was going to meet in the middle somewhere. Maybe ill just stick with the 60:40.....

I'm no expert but it sounds like you are on the right track. I think you have done enough research and should follow your instincts. It sounds like you are going to have some very happy plants growing. You'll always have weeds but they'll be easy to manage on a raised bed. Some people just put newspapers on the ground and dump soil on top to kill whatever is there. Beneficial worms can eat through newspaper. Last year I just dumped soil on top of weedy grass in a new bed and hardly saw any competition coming through, thought it probably depends on the weeds. Loam sounds nice, though like someone else said, loam itself is a variable so no one feels comfortable telling you if loam will work as a amendment to compost without knowing what's in this loam. I used a mix with soil, imported compost and peat moss which works well enough but probably could be better. There's always better. I will definitely mulch this year to help keep the soil moist while still able to breathe. It requires some patience and diligence to keep beds moist during the summer without an automated system.

Wow wasn't expecting so many replies. All the info is very helpful thank you everyone. I think I'm going to nix the weed block, leave the chicken wire and I'll just cover the bottom of the bed with yard waste leftover from the fall (leaves, sticks, twigs, grass clippings) before it gets filled so the worms can work their way in (and for extra compost).

I have always just used a combination of native soil and composted organic matter such as manure to fill raised beds. Mulching has kept the weeds from sprouting from any seeds that were in the soil. If you aren't going to use your native soil, I'd at least use a spading fork to loosen it and perhaps to turn in a bit of organic matter before putting down your wire mesh and adding the raised beds with amendments. That will create a less abrupt transition and I expect improve water movement and root transitions. I don't think I would use chicken wire for the mesh, however, since the two sizes sold around here have holes large enough to let in voles and it breaks down quite quickly. I'd use hardware cloth instead.